The Case of the Missing Carbon

Half the CO2 we pump into the atmosphere doesn’t stay there, which is good. But climatologists wish they knew where it’s going.

Climatologists have a rather large problem with their bookkeeping. Human beings release about 7 billion metric tons of carbon, in the form of carbon dioxide, into the atmosphere each year. (A metric ton equals 2,204 pounds.) Yet the amount of CO2 in the atmosphere rises annually by only 3.4 billion tons. Where does that remaining 3.6 billion tons go? This is not just a problem for climatological green eyeshades: the fate of the missing CO2 could bear directly on the course of global warming in coming decades.

Human-generated CO2 that doesn’t end up in the atmosphere has only two places to hide: land and sea. The ocean both soaks CO2 directly out of the air and spits CO2 back into the air. These two processes would tend to balance exactly, with no net movement of CO2 in either direction, were it not for the fact that some carbon is always escaping the loop by sinking to the bottom of the ocean. Until recently some researchers figured that just about all the missing carbon might be stored in the deep.

But in 1990 Pieter Tans, a geochemist at the National Oceanic and Atmospheric Administration, jolted his colleagues with some new calculations. Most of the missing carbon must be stored in the Northern Hemisphere, Tans figured, because that’s where most of it is produced by smokestacks and tail pipes, and the interhemispheric winds aren’t strong enough to spread a lot of it south of the equator. Yet the pressure on CO2 to leave the atmosphere for the northern oceans--as evidenced by the measured concentrations of the gas in air and sea--is not very great. Tans’s computer model suggested that the ocean is absorbing no more than 1 billion tons of CO2 each year. The remaining 2.6 billion, he said, must be hiding out on land, probably in forests in the temperate regions of the Northern Hemisphere.

Other researchers have since pointed out some flaws in Tans’s model. For example, he neglected the effects of evaporation, which cools the top millimeter of water by half a degree. Even this astonishingly thin skin lets the ocean absorb an extra 700 million tons of CO2. (The cooler the water, the better it absorbs CO2, which is why you should always keep soda cold.) With revisions such as this one, Jorge Sarmiento, an ocean modeler at Princeton, has run a more complex simulation of the ocean’s absorption of CO2, and he concludes that it takes in 2 billion tons a year. Still, that leaves another 1.6 billion tons unaccounted for.

Climatologists are now trying to find that carbon on terra firma. Plants on land are constantly taking up CO2--around 100 billion tons a year, researchers estimate--and converting it into organic matter through photosynthesis. But like the ocean, the land gives CO2 back: when plants die, microbes in the soil degrade their organic matter to CO2 again. The key question, then, is whether the plants, especially in northern forests, have somehow gotten ahead of the microbes and are converting CO2 into wood and other organic matter faster than microbes can send it back. The carbon might be getting stored in living trees--that is, the biomass of the forests might be increasing from year to year--or it might be getting stored in the forest soil, in the form of organic matter (leaves and branches and the like) that is not completely degraded.

Columbia University climatologist Aiguo Dai thinks the plants have gotten ahead of the microbes--because of a change in the weather. The productivity of plants and the efficiency of soil microbes both depend on precipitation and temperature, and in recent decades, says Dai, the weather has favored the plants. Dai recently ran a computer simulation based on 50 years’ worth of meteorological records. He found that the northern forests could be soaking up about half the 1.6 billion tons a year of missing CO2.

Other researchers suspect that air pollution is boosting plant growth. Nitrogen compounds from the burning of fossil fuels might be acting as fertilizer, and the bigger supply of CO2 itself might be making plants grow faster. Kevin Harrison and Wallace Broecker, both geochemists at the Lamont-Doherty Earth Observatory in New York, have simulated this fertilization effect. Their results indicate that up to 60 percent of the missing CO2 may be hiding in about equal portions in plants and soil.

Such simulations, however, are more suggestions than proofs. Researchers are painfully aware of their need for solid evidence that plants have actually stored large amounts of CO2. In one attempt to get it, researchers at the Finnish Forest Research Institute in Helsinki last year reviewed forestry surveys. They estimated that European forests increased, in terms of sheer volume of wood, by 25 percent between 1971 and 1990. To calculate how much CO2 had been drawn out of the atmosphere, the researchers also added the lumber that had been harvested from the same forests during the same period. They concluded that European forests have been absorbing as much as 120 million tons of carbon a year, and they suggested that similar biomass accumulation should have occurred also in other continents.

If so, then northern forests really would be a sink for half or more of the carbon that’s missing from the global budget. One prominent ecologist, though, has recently heaped scorn on such claims. Richard Houghton of the Woods Hole Research Center in Massachusetts points out that European forests, which have been harvested for centuries, are made up of young, fast-growing trees. Many of the forests of Canada and Siberia, on the other hand, have hardly been touched; as a result they are mature, slow-growing, and unable to capture much CO2. What’s more, says Houghton, the Finns’ tally ignored ways in which even the European forests give back CO2 to the atmosphere. Stumps and twigs left behind during timber harvests rot, for example, and timber products also release CO2 over the course of years. When Houghton adds up both the estimated CO2 gains and losses from all northern forests, he gets a figure close to zero.

Houghton is gloomy about finding the missing CO2. If 1.6 billion tons a year of CO2 can’t be accounted for today, he estimates that since 1850, when the industrial revolution began to have global effects on the atmosphere, some 100 billion tons of CO2 have gone missing--an amount equal to the biomass of northern temperate forests. If it were all stored in those forests, even split between the soil and the plants, we ought to be walking into it. What might be happening instead is that it’s everywhere, explains Houghton--in the tropics, in the Southern Hemisphere, in the high latitudes, and perhaps even some extra still undetected in the ocean. If that’s the case, the increase in any one place may well be too small to measure. That would be sad, but maybe that’s what nature’s done to us, says Houghton. The carbon is there, but nobody can find it.

Yet he and other researchers agree that there’s an urgent need for them to find it. It’s perhaps the most important problem that we have to solve, says Sarmiento. Time is of the essence because no one knows what effect the greenhouse effect itself is going to have on whatever process is storing away the extra CO2. Perhaps the plants that are capturing most of the carbon will capture even more as Earth gets warmer. Or perhaps they will disappear as the warmer temperatures rearrange ecosystems. For now, something is cushioning us from the full brunt of global warming. It would be nice to know if that cushion is about to be pulled away.